Your MIDI tracks have embedded volume/pan controls. Select one of your tracks and press F2 to open an event list for the track. Look near the beginning of the event list for "VolCtrl: 7" and/or "PanCtrl: 10". You will need to delete these events if you want to use the mixer window to control volume and panning. You will probably need to do this for each of your MIDI tracks; if so, a quicker way is to select all of your MIDI tracks and press F2 to open an event list for all of the tracks. This information also applies to reverb (ReverbCtrl: 91), and chorus (ChorusCtrl: 93). If you have imported a MIDI file from Band-in-a-Box®, the MIDI file will have these controllers, unless you disabled the saving of volume/reverb/chorus/panning controls in Band-in-a-Box®.